1. Field of the Invention
The present invention relates generally to the production of non-metallic honeycomb structures for use in situations where high thermal conductivity through the structure is required. More particularly, the present invention relates to a novel and improved corrugation process for producing thermally conductive honeycomb structures from non-metallic composite materials and products thereof.
2. Description of Related Art
Honeycomb shaped materials have long been used to increase the relative stiffness and strength of a structure without imposing a corresponding increase in weight. In particular, honeycomb structures have been incorporated as core materials in sandwich constructions having dense, high strength facings. Such structures provide the highest stiffness to weight ratio of any common materials design. Accordingly, these lightweight constructs are widely used in military and commercial aircraft, automotive bodies, engine components, recreational equipment, marine craft, support structures and cargo containers.
Honeycomb core materials are products consisting of thin sheets which are attached in such a way that numerous cells are formed. While various honeycomb configurations have been employed for different purposes, most honeycomb structures consist of a nested array of hexagonal cells which tend to provide the best characteristics overall. Along with cell configuration, the properties of a honeycomb core are largely determined by cell density and the material of manufacture. Higher cell density leads to better mechanical properties of a piece, but at the cost of increased weight. However, weight limitations and other problems can often be alleviated by the selection of appropriate materials.
The search for structural materials with desirable properties has resulted in the extensive development of non-metallic composite materials for honeycomb structures due to their light weight and anti-corrosive properties compared to metallic honeycomb structures. Composites are materials in which two or more distinct substances such as glasses, ceramics, or polymers are combined to produce a material with structural or functional characteristics different from the individual constituents. The constituents retain their individual characteristics and are distinguishable on a microscopic scale. Typically one constituent is classified as the reinforcement and the other as the matrix. The reinforcement generally provides the strength or stiffness in the composite while the matrix binds the reinforcement together and contributes to the distribution of the load.
Generally two major classes of polymers are employed in composite materials and may be classified as thermosetting and thermoplastic. The principal differences between the two polymer classes is the degree of crosslinking and response to elevated temperature. Thermosetting resins or polymers are extensively crosslinked and undergo irreversible changes when heated or reacted with a selected catalyst or a curing agent. Examples of important thermoset matrix materials are polyesters, epoxies, polyimides and phenolics. In contrast, thermoplastic materials are generally not crosslinked and soften as they are heated. After being exposed to heat they return to their original condition when cooled below their melt temperature. Common thermoplastic materials include polyolefins, vinyls, polyamides, acrylics, polycarbonates, and polysulfones.
Thermoplastic systems have advantages over some of the thermosets in that no chemical reactions which cause release of gas products or excess thermal heat are involved. Further, they are generally more ductile and tougher than thermosets. Processing is limited only by the time needed to heat, shape, and cool the structure. In contrast, thermoset resins undergo an irreversible chemical reaction or cure in the presence of a catalyst, heat, radiation and/or pressure. Once cured they cannot be returned to the uncured state and can no longer flow. However, they tend to exhibit better chemical resistance, adhesion and superior electrical characteristics when compared to thermoplastics. While thermosets have traditionally been the principal matrix for composites due to their relative ease of handling and processing as well as low unit cost, thermoplastic matrices are becoming more popular for a number of applications due to new formulations. For example some high performance thermoplastics now equal the most common thermosets in temperature capability.
Similarly, advances in the mechanical properties of reinforcement materials have expanded the use of composites in honeycomb fabrication. Selection of the type and form of reinforcement will vary in accordance with the design requirements for the structure. General criteria for a desirable reinforcement include high strength, high modulus, low weight, low cost, ease of fabrication and environmental resistance. Common materials having some or all of these properties and useful for fabricating reinforcements are glasses, polymers, ceramics and graphite each of which may come in many different forms. Widely used forms of reinforcements include continuous fibers or filaments, chopped fibers, mats, woven fibers, particles or ribbons. While different forms of reinforcement are used in different applications, fibers have been used most extensively for the development of advanced composites as they provide the highest strength and modulus per unit weight. Moreover they may be woven, chopped or used as mats depending on the desired properties of the structure.
In any case, honeycomb structures are typically manufactured using an expansion process or a corrugation process depending on the nature of the material employed and the cell configuration desired. In the expansion process sheets of the desired reinforcement material are cut to the desired shape and strips of adhesive are applied. Quite often the adhesive material is actually printed on the sheet material before cutting. The cut material is then stacked in layers and bonded at the selected adhesive points. Usually the alternate sheets have the position of the applied adhesive staggered to provide the correct shape upon expansion. The bonded stackup is then cut to the desired configuration and mechanically expanded. In non-metallic honeycomb the expanded stackup is often impregnated with a thermoplastic or thermoset matrix and cured to retain this configuration. The corrugation process for manufacturing non-metallic honeycomb often involves the use of sheet-like preimpregnated reinforcement material (prepreg). Prepregs are obtained by impregnating fiber, fabric or paper reinforcement material with a thermoplastic or thermoset matrix. In one example of this method the single or multi-ply prepreg material is passed through mated corrugating rollers to form a corrugated sheet. Other methods, such as vacuum formation or pressure molding, may also be used to fabricate these structures. The corrugated sheets are then stacked and aligned to form a honeycomb arrangement and cured or otherwise bonded to one another at the appropriate nodes to form a honeycomb panel.
The use of corrugation techniques to produce honeycomb structures is well known in the art. For instance, U.S. Pat. No. 5,030,305 (withdrawn) describes a method of manufacturing reinforced thermoplastic honeycomb structures. The fiber reinforcement may be in the form of a woven or nonwoven web which is corrugated using vacuum, molding or roller methodology. Thermoplastic resin may be introduced in the form of staple fibers blended into the nonwoven web, by melt coating the web, or by laminating a preformed thermoplastic resin film to the web. The sheets thus formed may be stacked and heated at selected points to produce the desired honeycomb structure.
Honeycomb structures formed of thermoplastic and thermoset composite materials are particularly attractive in light of their ease of manufacture and inherent physical properties. For example, fiberglass honeycomb materials have been in use for more than forty years as cores for radomes and antenna windows due to their beneficial electrical characteristics. Similarly, in addition to low weight and high strength, non-metallic honeycomb structures often act as good thermal insulators and are frequently used in this capacity by industry. In particular carbon-carbon and carbon-phenolic materials have been developed for thermal protection systems. One important use for these materials is in the leading surfaces of aircraft which are subjected to high aerodynamic heating loads due to atmospheric friction. The use of insulating honeycomb structures with dead air spaces allows the heat to gradually dissipate without adversely affecting the interior environment of the aircraft.
While the insulating properties of non-metallic honeycombs are desirable in many instances, there are situations where it is advantageous to have high strength, lightweight materials which have a high thermal conductivity. For example, jet aircraft engines require a high degree of thermal transfer through the engine structure in order to maintain structural temperature loads at acceptable levels. Accordingly, the engine structure from the combustion chamber to the outer nacelle must function as a thermally conductive heat sink while still being extremely strong and lightweight.
Prior art honeycomb structures made from aluminum have been shown to have the required strength along with sufficient thermal conductivity to allow the necessary transfer of heat from the core flow path to the fan bypass flow path or in nacelle and/or thrust reverser structures. However, aluminum is subject to corrosion, thermal expansion and associated stress problems. To avoid these complications, various glass fiber reinforced composite honeycomb structures and polyacrylonitrile (PAN) based carbon fiber reinforced composite materials have been suggested as potential substitutes for the aluminum honeycomb structures in jet aircraft engines. However, such non-metallic honeycomb structures are generally not suitable due to their poor thermal conductivity.
One solution to this problem of low thermal conductivity in non-metallic honeycomb structures has been advanced in U.S. Pat. No. 5,288,537 which is owned by the same assignee as this invention and which is incorporated herein by reference. The application provides a strong, lightweight, nonmetallic honeycomb structure exhibiting a high degree of thermal conductivity. These novel honeycomb structures were based on the surprising discovery that highly conductive pitch based carbon fibers could be woven into non-metallic composite reinforcement materials to provide the desired levels of thermal conductivity. As the conductive carbon fibers can be integrated with a variety of woven reinforcement materials having different properties, thermally conductive honeycomb structures may be designed and fabricated to exhibit the desired mechanical characteristics.
More specifically the walls of the thermally conductive honeycomb include a plurality of non-metallic fibers having low thermal conductivity in combination with a plurality of non-metallic fibers having high thermal conductivity. Both sets of interwoven fibers are impregnated in a resin matrix which is then used to fabricate honeycomb structures using conventional production techniques. The disclosure further specifies that the thermally conductive fibers may be incorporated at any angle or oriented parallel or perpendicular to the lengthwise axis of the honeycomb structure. In a preferred embodiment of the invention the pitch based carbon fibers are angled relative to the lengthwise axis so as to provide additional mechanical strength to the thermally conductive structure. The resulting structures are strong, lightweight and exhibit a surprisingly high thermal conductivity.
While the heat transfer properties of such arrangements are a vast improvement over the prior art, structural considerations and manufacturing techniques impact the thermal characteristics of the honeycomb construct. For instance, in order to achieve honeycomb structures having maximum shear properties, the conductive fibers are preferably oriented with a 45.degree. bias prior to performing the corrugation process. This undesirably increases the path length for heat transfer thereby increasing the insulating capacity of the honeycomb structure. Further, the process of weaving the brittle, thermally conductive fibers into the reinforcement can result in fiber breakage, thereby compromising the structural and thermal integrity of the resulting material. As with the increased path length, this introduction of fiber discontinuities raises the heat capacity of the honeycomb structure with a corresponding decrease in thermal transfer capability. In addition, the fabrication of the interwoven thermally conductive reinforcement fabric may undesirably complicate the production of the honeycomb structure by increasing the number of manufacturing steps along with the unit cost.
Accordingly it is an object of the present invention to provide strong, lightweight, non-metallic honeycomb structures which exhibit relatively high thermal conductivity.
It is another object of the present invention to provide an efficient, cost effective process for the fabrication of a strong, lightweight, thermally conductive non-metallic honeycomb structure.
It is yet another object of the present invention to overcome the aforementioned problems with the prior art techniques for forming a thermally conductive honeycomb structure.
It is still another object of the present invention to provide thermally conductive non-metallic honeycomb structures for use in applications where the transfer of thermal energy is necessary, such as in aircraft engine structures.